1. Field of the Invention
The present invention relates to an image forming apparatus and particularly relates to an image forming apparatus having a fixing unit with improved fixing characteristic.
2. Description of the Related Art
Electrophotographic apparatus including electronic copiers and laser beam printers, etc. are provided with a fixing unit where a developer image formed on a photosensitive body and then transferred onto an image bearing medium is fixed on the image bearing medium by heat. Such a fixing unit comprises a pair of rollers to catch and feed the image bearing medium and at least one of the rollers is a heating roller. For improvement of the fixing characteristic of such heat fixing unit in electrophotographic apparatus, it is necessary to extend the fixing time. To extend the fixing time, the rotation speed of the pair of rollers may be made lower to slow down the fixing speed or the nip width by which the pair of rollers catch the image bearing medium may be enlarged. The nip width may be enlarged by larger outer diameter of the rollers, lower rubber hardness on outer surface of the rollers, increased roller pressure or thicker rubber.
When the fixing speed is slowed down for a longer fixing time, the operation speed of the process unit in the whole electrophotographic apparatus must be made lower, which results in a slower speed for image formation.
On the other hand, when the nip width is made larger for a longer fixing time, it involves the drawbacks as listed below:
1. Larger outer diameter of the rollers results in a larger fixing unit, which leads to a higher cost;
2. Lower rubber hardness tends to result in wrinkles on the image bearing medium;
3. Larger pressurizing force at rollers causes larger distortion at the roller shafts, which results in lower feeding performance for the image bearing medium; and
4. When the outer diameter of the rollers is kept constant, thicker rubber necessarily causes smaller shaft diameter. This leads to lower shaft rigidity causing larger distortion at the shaft, which results in lower feeding performance for the image bearing medium. On the other hand, when the shaft diameter is kept constant, thicker rubber results in larger outer diameter of the rollers, which leads to a larger fixing unit. Besides, a fixing unit for color electrophotographic apparatus requires a rubber layer coating on the heat roller. This means that thicker rubber results in a larger temperature difference between the roller surface and the shaft. In this case, when the surface temperature is raised to a required value, the shaft is inevitably heated up to an unusually high temperature, which deteriorates the bonding strength between the shaft and the rubber layer and may result in separation of the rubber layer.